The ignition capacitors are somewhat standardised in terms of capacitance (usually 0.1 to 0.3 mFd) but vary somewhat in packaging and mounting provisions. The penalty for using the wrong capacitance, within that range, is that one or other contact will pit while the other one develops a corresponding mountain. Usually small engines do not run enough hours for this to be an issue. Hence the use of a capacitor from another broadly similar magneto should work. (Famous last words, of course - you may turn out to have some oddball ignition system I haven't heard of.)
